Apparatus for treating air with electrically charged and uncharged liquid sprays

ABSTRACT

Apparatus for treating gas or vapor includes an impeller arranged in a conduit which draws dusty or fume-laden air through the duct. One or more nozzles are arranged to spray fine droplets of water onto the impeller. Downstream of the impeller, a set of angled collection plates collect the fine water droplets emitted by the nozzle, together with dust or fumes collected by the droplets. The nozzle is connected to a high voltage source, so that the droplets emitted by the nozzle are electrostatically charged. This causes them to attract dust particles and other contaminants. The collection plates are earthed, so that the charged droplets are attracted to them, improving the effectiveness of the apparatus.

This application is a divisional of application Ser. No. 08/274,741,filed Jul. 14, 1994, now U.S. Pat. No. 5,518,525.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for treating a gas orvapor. In particular, the method and apparatus have application incontrolling dust in air used for ventilation.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of treating gas orvapor comprising the steps of:

conducting gas or vapor through a conduit;

atomising a liquid to create a spray of fine droplets of the liquid;

electrostatically charging the droplets; and

directing the spray into the conduit to cause dispersion of the dropletsthrough the gas or vapor, so that dust particles or other predeterminedcomponents in the gas or vapor are attracted to the droplets.

The method may be applied to the extraction of dust and fumes fromventilation air, in which case the dispersion of the liquid throughcontaminated air causes the liquid droplets to collect dust, fumes, orparticulate matter in the air.

The droplets may be charged by applying a high voltage to a nozzle fromwhich the droplets are discharged.

The method may include conducting the gas or vapor in the conduit past aplurality of collector plates which are maintained at a differentpotential from that applied to the droplets, so that the droplets areattracted to the collector plates.

The method may include the steps of providing a rotating surface in theconduit, and directing the spray at the rotating surface.

The method may include directing an auxiliary spray of fine dropletsover an entrance opening of the conduit, thereby to prevent ignition ofexplosive gas or particles outside the conduit.

The fine droplets are preferably less than 1 mm in diameter.

Further according to the invention apparatus for treating a gas or vaporcomprises:

a conduit arranged to conduct gas or vapor therethrough;

at least one nozzle arranged to be supplied with liquid under pressureand to emit a spray of fine droplets of the liquid into the conduit,thereby to cause dispersion of the droplets through the gas or vapor inthe conduit; and

a high voltage source connected to the at least one nozzle forelectrostatically charging the droplets which are emitted therefrom.

The at least one nozzle may be connected to a source of liquid underpressure by a pipe having an electrically insulating portion between thenozzle and the source.

The high voltage source may be a DC source with an output voltage in therange 5 to 50 kV, preferably about 20 kV.

The nozzle may be connected via a first pipe to a source of a liquidsuch as water, with a second pipe for compressed gas connected to thenozzle so that application of the liquid and the compressed gassimultaneously causes a spray of atomised droplets of the liquid to beemitted from the nozzle, the nozzle being electrically insulated fromboth the first and second pipes.

An impeller may be disposed within the conduit for conducting the gas orvapor therethrough.

Preferably, a plurality of collector plates are disposed downstream ofthe nozzle in the conduit, the plates being maintained at a differentpotential from that applied to the nozzle so that the droplets areattracted to the collector plates.

An auxiliary nozzle may be arranged to direct an auxiliary spray of finedroplets over an entrance opening of the conduit, thereby to preventignition of explosive gas or particles outside the conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of exampleonly with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic side view of an air treatment apparatusaccording to the invention;

FIG. 2 shows a sectional plan view of part of the apparatus shown inFIG. 1; and

FIG. 3 is a schematic end view of an alternative embodiment of theapparatus.

DESCRIPTION OF PREFERRED EMBODIMENTS

Dust extraction apparatus according to the invention is shown generallyat 10 in FIG. 1. The apparatus comprises a conduit in the form of asection of ducting 12. Air is caused to pass through the ducting in thedirection of arrows 14. The air may be hot or cold, and may containdust, gas or vapor. The ducting 12 houses heat exchange apparatus whichcan be operated to transfer heat from the air passing through theconduit to a source of liquid, about which more is said below.

In some applications, the ducting may be dispensed with, and the conduitmay simply comprise a tunnel or the like, for example in a mine working.

At the upstream end of the ducting 12 a source of liquid (usually water)is connected to an inlet pipe 16, which terminates in a nozzle 46.Associated with the inlet pipe 16 is a compressed air line 18. The line18 may carry compressed air, gas or steam which breaks the inlet liquidcarried by the pipe 16 into a spray 20 of fine droplets. The nozzle 46is designed so that the droplets have a mean diameter of less than 1 mm.The small size of the droplets increases the available surface area ofliquid for heat exchange and dust extraction applications.

The spray 20 is directed onto an impeller 22 which is driven rotatablyby a motor 24. The impeller is arranged transversely across the duct,and the outer edges of the impeller blades are parallel to the walls ofthe duct. The inner corners of the blades nearest the source of thespray 20 are cut off diagonally to define a conically shaped recess 21into which the spray 20 is directed. The motor 24 is suspended insidethe duct 12 by brackets 26. The motor can be an electric, pneumatic orhydraulic motor, for example.

It will be appreciated that the described motor and impellerconfiguration is entirely exemplary, and that numerous variations arepossible. For example, in some dust extraction and chemical processes itmay be preferable for the duct 12 to be vertical, with the motorattached in a vertical position.

A sump 28 is provided at the bottom of the ducting 12 and has anoverflow pipe 30. A high pressure pump 32 draws water from the sump anddirects it to a delivery nozzle 34. The nozzle 34 is arranged to directliquid into the conically shaped recess 21 at the centre of theimpeller. The high pressure pump 32 and nozzle 34 recirculate the liquidfrom the sump to allow repeated heat exchange passes between the liquidin the sump and the air 14 being drawn through the ducting.

Means may also be provided for recirculating some of the air, gas and/orvapor passing through the ducting 12 so that repeated extraction and/orheat exchange passes may occur. The recirculation of this air, gas orvapor is shown schematically by the dotted line 36 at the top of FIG. 1.The recirculation may occur through an adjacent mine shaft, stope, orworking area, for example.

At the downstream end of the ducting 12, liquid collecting means 38 areprovided. These means comprise a series of spray eliminator plates 40which act as heat exchange surfaces and collect droplets of liquidinside the ducting 12. The droplets of liquid which accumulate on thespray eliminator plates run down by gravity to fall back into the sump28. If the apparatus is used in an air cooling application, theevaporation of the falling droplets supplements the cooling of the air.The orientation of the spray eliminator plates can be seen more clearlyin the sectional plan view of FIG. 2. It will be seen in this figurethat the plates are arranged at an oblique angle relative to thelongitudinal axis of the ducting 12. The plates are arranged in sets,with alternate sets having their plates arranged at approximately 90° toeach other. Plates 42 near the exit of the liquid collecting means areprovided with lips 44 to catch any droplets which might otherwiseescape, which would reduce the heat transfer efficiency of theapparatus.

In order to improve the dust and fume extraction capabilities of theapparatus, a high voltage DC source 48 is provided, which generates ahigh output voltage in the region of 5 to 50 kV, typically 20 kV. Thisoutput is connected via an insulated conductor 50 to both spray nozzles34 and 46, as shown schematically in FIG. 1. The nozzles are metallic,so that they impart the high voltage from the source 48 to the dropletsemitted in use, charging them electrostatically. In the case wheremetallic pipes feed the nozzles, tubular insulating spacers 52 and 54 ofa plastics material having a suitably high dielectric co-efficient areprovided to interrupt the pipes, which otherwise would earth thenozzles.

In a prototype installation, a high voltage source similar to aconventional electric fence energiser was utilised.

The spray eliminator plates 40 are connected to the same earth as thehigh voltage supply 48, so that the electrostatically charged waterdroplets, with their burden of dust particles or fumes, tend to beattracted towards the plates for collection. The electrostatic charge onthe droplets causes them to attract fine particles as they move towardsthe plates, so that the effectiveness of the apparatus is significantlyincreased.

As mentioned above, the apparatus 10 of the invention may be used in aheat exchange application. In this case, the source of liquid beingintroduced into the ducting by inlet pipes 16 is provided at atemperature towards which the air is to be brought. In particular, theillustrated apparatus may be used in a heat exchange process for heatrejection from an underground refrigeration plant in a mine. Here, theinlet pipe 16 would carry high temperature cooling water from therefrigeration plant to lose heat to relatively cooler return or rejectventilation air 14 in the duct 12. The spray of fine droplets providedby the compressed air line 18 would be used to create a mist of finedroplets to increase the surface area of liquid available for heatexchange and improve the removal of heat, dust and fumes, typicallydiesel and blasting fumes. If the illustrated apparatus were to be usedin mine stopes or development ends in an underground mining operation,the duct 12 (or simply a tunnel) would be carrying relatively warm orhot ventilation air which would be cooled by cold service water fed inthrough the inlet pipe 16. Once again, the compressed air fed in throughthe line 18 would be used to create a mist of fine droplets to improveheat exchange and the removal of heat, dust and fumes.

In order to prevent possible ignition of explosive gases or dust by thehigh voltage circuit of the apparatus, an auxiliary wide-angle spraynozzle 56 is located at the entrance of the ducting 12, and creates afine mist of water between the exterior and the interior thereof.

The heat exchange, and dust and fume removal apparatus described abovecan be used in various applications. Valuable dusts containing platinumor gold may be collected at processing plants and assay offices by theapparatus of the invention. In a similar way, dangerous silica, coal orasbestos dust at crushers may be removed. (Collected damp dust isusually safer for disposal than dry dust). The liquid sprays of theapparatus can also be used to keep the gases or vapours below theirignition temperatures, enabling potentially explosive gases such asmethane or vapors such as ether vapors to be safely extracted. Theapparatus can also be used to add or remove chemicals into a gas flowduring a manufacturing process.

FIG. 3 shows in a highly simplified schematic form an embodiment of theapparatus in which a conduit is divided into a number of subconduits,each with its own section of ducting 12, with an associated impeller 22and spray nozzles 34 and 46. The sets of nozzles 34 and 46 can beconnected to respective common sources of liquid under pressure. The useof a multi-conduit arrangement allows a plurality of standard-sizedunits according to the invention to be provided, instead of requiringdifferent-sized units for different applications.

The described invention occupies a relatively small volumetric space,has a high heat transfer and dust collecting capacity, and may beretrofitted into an existing ducting line.

I claim:
 1. Apparatus for treating a gas or vapor comprising:a conduitarranged to conduct gas or vapor therethrough; at least one main nozzlearranged to be supplied with liquid under pressure and to emit a sprayof fine droplets of the liquid into the conduit, thereby to causedispersion of the droplets through the gas or vapor in the conduit; ahigh voltage source connected to the at least one main nozzle forelectrostatically charging the droplets which are emitted therefrom; andan auxiliary nozzle arranged to direct an auxiliary spray of fineuncharged droplets over an entrance opening of the conduit, thereby toprevent ignition of explosive gas or particles outside the conduit. 2.Apparatus according to claim 1 wherein the at least one main nozzle isconnected to a source of liquid under pressure by a pipe having anelectrically insulating portion between the nozzle and the source. 3.Apparatus according to claim 1 wherein the high voltage source is a DCsource with an output voltage in the range 5 to 50 kV.
 4. Apparatusaccording to claim 3 wherein the output voltage is approximately 20 kV.5. Apparatus according to claim 2 wherein the at least one main nozzleis connected via a first pipe to a source of a liquid, with a secondpipe for compressed gas connected to the at least one main nozzle sothat application of the liquid and the compressed gas simultaneouslycauses a spray of atomized droplets of the liquid to be emitted from theat least one main nozzle, the at least one main nozzle beingelectrically insulated from both the first and second pipes. 6.Apparatus according to claim 1 wherein an impeller is disposed withinthe conduit for conducting the gas or vapor therethrough.
 7. Apparatusaccording to claim 1 wherein a plurality of collector plates aredisposed downstream of the at least one main nozzle in the conduit, theplates being maintained at a different potential from that applied tothe at least one main nozzle so that the droplets are attracted to thecollector plates.